The present invention relates to a low-temperature storage including a storage chamber cooled/maintained in an ultra low temperature range around −80° C. by a cooling device comprising a binary refrigeration circuit which realizes a lower storage temperature by cascade connection of an evaporator in a refrigeration circuit on a high-temperature side to a condenser in a refrigeration circuit on a low-temperature side. The present invention relates particularly to a partition for dividing a front surface opening of an insulation box into upper/lower openings, a partition member for dividing a frontage into upper/lower frontages, and a latch structure of inner doors, capable of selecting two or four inner doors in accordance with the partition member.
Among low-temperature storages, there is a shed in which a binary refrigeration circuit is used in order to produce an ultra low temperature range of about −60° C. to −90° C. and which is constituted by cascade connection of an evaporator of a refrigeration circuit on a high-temperature side to a condenser of a refrigeration circuit on a low-temperature side. This low-temperature storage is sometimes referred to as an ultra low temperature freezer by its temperature band. To more efficiently store a storage object, the inside of a storage chamber is divided into upper/lower chambers by a plurality of shelves. When the storage chamber divided into a plurality of chambers is opened/closed with one door, outside air entirely enters the storage chamber with the opening/closing of the door. Therefore, the outside air also enters a medium storage chamber corresponding to the shelf unrelated to an object to be discharged/charged. To avoid this, a plurality of inner doors have been disposed corresponding to the medium storage chamber. In this case, the front surface opening of the insulation box is divided into two upper/lower openings by a partition, and upper/lower inner doors are disposed. This medium storage chamber is sometimes further divided into upper/lower small storage chambers by the shelf.
For example, as described in Japanese Patent Application Laid-Open No. 6-129760, there is a refrigerator whose storage chamber is divided by a partition portion and in which a partition front frame is also disposed in a front surface portion of the partition portion and in which a space formed by the partition front frame and the partition portion is filled with a foamed insulating material. The storage chamber divided into upper/lower chambers by the partition portion is usually openably closed by insulating doors. A gasket provided with a magnet is disposed on an inner surface peripheral edge portion of each door, and this gasket adheres to a metal cover disposed for an opening edge of the insulation box and the front surface of the partition front frame to prevent permeation of the outside air into each storage chamber. For example, as described in Japanese Patent Application Laid-Open No. 2001-183052, there is a cold storage in which the front surface opening of the storage chamber is openably closed with one thermal insulating door, and a plurality of resin upper/lower inner doors are openably/closably disposed inside the thermal insulating door. Especially the inner door mechanically closes the opening by a latch mechanism.
In the household refrigerator described in the former document, the medium storage chamber divided by the partition is not further divided into upper/lower small sections (small storage chambers) by addition of an optional component or change of the inner door. In a low-temperature storage such as a business refrigerator, an inner volume is usually enlarged as compared with the household storage. Therefore, the optional component can be preferably added at any time in accordance with user's (so-called customer) demand. In the cold storage described in the latter document, a detailed structure of the latch mechanism is unknown. Judging from the drawing, the thermal insulating inner door is forcibly pushed into the storage chamber using a principle of leverage by rotation/operation of the latch mechanism, and the opening is firmly and securely closed by the inner door. However, in the ultra low-temperature storage for use in an ultra low temperature range of about −60° C. to −90° C., the outside air which has entered by the opening/closing of the inner door is rapidly cooled. Therefore, a frosting phenomenon easily becomes remarkable in the vicinity of an outlet/inlet, particularly into an in-shed wall surface of the inner door provided with the latch mechanism on a non-shaft support side. Since thickness and hardness of frost gradually grow by long-term storage in the frosting phenomenon, the grown frost hinders a latch operation of the latch mechanism. That is, in the beginning of the frosting phenomenon, the inner door can be pushed inwards even slightly forcibly by the principle of leverage, but it is also expected that the thickness grows to such an extent that the inner door cannot be closed during long-term use. In this case, the latch mechanism is operated with a considerably strong force, and there has been a disadvantage that a part (especially a portion lacking in strength) of the latch mechanism is deformed by a function of a stress which is not less than a strength (withstand load) in the latch mechanism, and the mechanism does not normally operated thereafter.